Information Security

Creating an encrypted disk image on a Mac

How to create an encrypted disk image on a Mac, as a secure container for files.

The steps here show the basic outline, but Apple change the details with each new version of the Operating System. Take these steps as a guide.

  1. Open Disk Utility found in the Utilities folder (/Applications/Utilities).
  2. Click the New Image button, or choose New then Blank Disk Image from the Disk Utility File menu.
  3. Enter a name in the Save As: field. This name is used for the disk image (.dmg) file.
  4. Change the save destination if you wish to.
  5. Mac OS X encrypt disk image window
  6. Change the volume name to match the disk image name.
  7. Select a size for the image file from the Volume Size drop-down menu, or use Custom to set the space you require.
  8. The default Mac OS X Extended (Journaled) volume format will be correct in most cases, if you wish to change it use the drop-down menu.
  9. Choose an image format. You can use sparse disk image for a disk image that only uses as much space as it needs, rather than a set amount of space. If you're not sure use the read/write disk image choice.
  10. From the Encryption: drop-down choose 256-bit AES if available, otherwise choose 128-bit AES to encrypt the image's contents with a password.
  11. Click the Create button.
  12. Enter and verify a good password in the dialog window that appears. This password will be saved in your keychain by default, it is recommended that you deselect this.  Note: If you forget this password then the files stored within the disk image will be inaccessible.
  13. Click OK.